The presently most widely used process for producing glass sheets, is a so-called tin bath float process wherein prescribed materials are melted in a melting tank, then introduced onto a molten metal tin bath in a reducing atmosphere at a temperature at which the viscosity of glass becomes about 10,000 poise, permitted to spread and move in the longitudinal and transverse directions by means of a mechanical external force and gradually cooled to around the glass transition temperature, to obtain a flat plane glass having a smooth surface. By this process, the smoothness of the product is remarkably improved as compared with the previous roll out process or the like, whereby a polishing step which used to be required, is no longer necessary.
However, this process also has some drawbacks and problems, and improvements are still desired. Namely, in the tin bath float process, a large quantity of tin is used, whereby exhaustion of the tin resource which is not abundant, is feared, it is necessary to maintain the atmosphere to be reducing by means of hydrogen gas in order not to let the metal tin be oxidized, accordingly, applicable fining agent is rather limited, the installation is obliged to be of a large size in view of a problem such as heat balance, whereby the installation investment tends to be large, tin is likely to penetrate into the glass from the surface in contact with the tin, whereby the quality of the product tends to be affected, it is weak against vibration such as earth quake, and it takes time for recovery of the production after an earth quake, and a large quantity of energy is consumed by heating and warming the glass.
Whereas, a production method such as so-called fusion method, has also been proposed, but such a method has not been satisfactory with respect to the surface smoothness of the product or the constant productivity and quality. Further, there has been a proposal such that a gas such as air is supplied from pores on the surface of a support, and molten glass is spread thereon to form a glass sheet (JP-B-50-36445). However, in order to directly and continuously supply the gas as proposed, an enormous quantity of the gas is required. Further, in order to pass it through pores, an extremely high pressure is required, whereby a stabilized control is extremely difficult. Thus, this method is not practical.
The object of the present invention is to solve the various drawbacks in the above mentioned existing processes for forming glass sheets.